topic_list: Don't show topic-list button in action sheet from app bar

Similarly to how we don't show the "Channel feed" button when
opening the same action sheet from a channel feed.

Author: chrisbobbe

lib/widgets/topic_list.dart

@@ -103,7 +103,10 @@ class _TopicListAppBarTitle extends StatelessWidget {
       child: GestureDetector(
         behavior: HitTestBehavior.translucent,
         onLongPress: () {
-          showChannelActionSheet(context, channelId: streamId);
+          showChannelActionSheet(context,
+            channelId: streamId,
+            // We're already on the topic list.
+            showTopicListButton: false);
         },
         child: Align(alignment: alignment,
           child: _buildStreamRow(context))));

test/widgets/action_sheet_test.dart

@@ -277,7 +277,6 @@ void main() {
       void checkButtons() {
         check(actionSheetFinder).findsOne();
         checkButton('Mark channel as read');
-        checkButton('List of topics');
         checkButton('Copy link to channel');
       }
 
@@ -413,17 +412,25 @@ void main() {
       });
     });
 
-    testWidgets('TopicListButton', (tester) async {
-      await prepare();
-      await showFromMsglistAppBar(tester,
-        narrow: ChannelNarrow(someChannel.streamId));
+    group('TopicListButton', () {
+      testWidgets('not visible from app bar on topic list', (tester) async {
+        await prepare();
+        await showFromTopicListAppBar(tester);
+        checkNoButton('List of topics');
+      });
 
-      connection.prepare(json: GetStreamTopicsResult(topics: [
-        eg.getStreamTopicsEntry(name: 'some topic foo'),
-      ]).toJson());
-      await tester.tap(findButtonForLabel('List of topics'));
-      await tester.pumpAndSettle();
-      check(find.text('some topic foo')).findsOne();
+      testWidgets('happy path from msglist app bar', (tester) async {
+        await prepare();
+        await showFromMsglistAppBar(tester,
+          narrow: ChannelNarrow(someChannel.streamId));
+
+        connection.prepare(json: GetStreamTopicsResult(topics: [
+          eg.getStreamTopicsEntry(name: 'some topic foo'),
+        ]).toJson());
+        await tester.tap(findButtonForLabel('List of topics'));
+        await tester.pumpAndSettle();
+        check(find.text('some topic foo')).findsOne();
+      });
     });
 
     group('ChannelFeedButton', () {